Ramp surface for an apparatus for coating tablets

ABSTRACT

A fluidized bed apparatus for applying a coating liquid onto the surface of particles includes a vertically disposed cylindrical product container having a peripheral wall, at least one cylindrical partition defining a centrally located up bed region and a peripherally located down bed region. A nozzle is centrally located through the orifice plate and is adapted to generate a spray of coating liquid upwardly into the up bed. A nozzle ramp is provided around the nozzle. The nozzle ramp is generally cusp shaped and includes nozzle-ramp surface. The nozzle ramp is centrally positioned around the nozzle and is directed upwardly towards the partition so that the nozzle-ramp surface directs particles moving generally horizontally across the orifice plate from the down bed upwardly into the partition and the up bed. The nozzle ramp may further include air passages for passing fluidized air upwardly to the nozzle-ramp surface.

This patent application is a divisional of U.S. patent application, Ser.No.: 09/715,855, filed Nov. 17, 2000 now U.S. Pat. No. 6,579,365, whichclaims priority from U.S. Provisional Patent Application, Ser. No.:60/166,799, filed Nov. 22, 1999, both of which are incorporated herewithin their entirety.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention generally relates to coating machines and, inparticular, to fluid-bed coating machines used for the coating oftablets.

B. Description of the Prior Art

Tablets are formed by pressing pharmaceutically active drugs, filler andbinding agents together. Once formed, it may be necessary, or desirableto provide the tablet with a coating which will:

1. prevent any portion of the drug from being released, such as in theform of dust;

2. mask any unpleasant odor or taste of the active drug, or any filleror binder used;

3. facilitate swallowing by providing a smoother and less absorbentouter layer;

4. protect the contents of the tablet from pre-mature digestion byproviding a coating which is resistant to gastric fluids;

5. control the rate of absorption of the drug by the small intestine;and

6. improve the appearance of the tablet and provide a printable surface.

The tablets are generally coated using machines which spray a coatingmaterial, such as hydroxypropylmethylcellulose onto the surfaces of thetablets while the tablets are in motion within a product container. Twocommon types of machines tumble tablets within a horizontally rotatabledrum during the spraying process, while another type of tablet coatingmachine uses a vertical flow of air to circulate tablets past avertically disposed spray nozzle. The prior art coating machines aredescribed below:

1. Dragee Kettle

For most applications, the exact thickness of the coated layer is notcritical and many different types of coating machines may be used toapply a crude, yet effective coating to the tablet. An older oncepopular type of coating machine is called a dragee kettle and examplesof these machines are disclosed in U.S. Pat. Nos. 3,831,262 and5,334,244. This machine includes a large drum-like vessel which istypically rotated about a horizontal axis. The vessel includes a coatingchamber which is partially filled with tablets to be coated so that asthe vessel rotates, the tablets roll and tumble along the inside wall ofthe coating chamber. During this tumbling motion, coating materials inthe form of aqueous or organic suspensions of liquids are sprayedthrough nozzles and into contact with the rolling tablets within thecoating chamber. During the coating process, a current oftemperature-controlled air circulates in the coating chamber of thedragee kettle, which helps evaporate the suspension agent of the coatingmaterial so that the coating material effectively dries and adheres tothe tablets.

One problem with the dragee kettle coating machine is that typically thetablets are not the only surfaces coated within the coating chamber.Even when a carefully controlled spraying schedule is followed (such asspraying at very short intervals while the dragee kettle rotates), muchof the sprayed coating material still ends up on the inside wall of thecoating chamber, as well as throughout the evaporation/venting ducting.This over-spraying creates numerous contamination and cleaning problems,and further increases the cost of the coating since much of the coatingmaterial is lost during the coating process.

The above-described dragee kettle type coating machine is limited tocoating tablets which do not require much precision in the thickness ofthe coated layer because the thickness of the coating of the tabletswill vary in the same batch. This process may be used to coat manydifferent types of pharmaceuticals, vitamins, and even candy, as long asuniform coating distribution and thickness are not required.

2. Perforated Pan

The next generation of tablet coating machines after the dragee kettleis called a perforated pall tablet coating machine. This machine hasimproved the tablet coating process and is the most common type oftablet coating machine in use today. The perforated pan machine includesa rotatable perforated drum which rotates about a horizontal axis withina housing, and further includes a plurality of nozzles positioned withinthe drum. The nozzles create a spray of coating material within the drumso that any tablets located within the drum will tumble about into andout of the spray pattern and, over a period of time, will accumulate acoating on their surface. An important improvement of the perforated pancoating machine over the dragee kettle is that the perforated panmachine allows air directed through the housing (using appropriateducting) to pass through the perorated drum and quickly reach thetablets tumbling therein. The perforations of the drum effectivelyexpose the tumbling tablets to the current of air, resulting in moreuniform distribution of drying air for each tablet. The drum furtherincludes solid baffles which are used to enhance mixing of the tabletbed in an effort to improve the distribution of the material beingsprayed onto the tablets.

3. Fluidized Bed Coating Machines

Another type of particle-coating apparatus is called a fluidized bedcoating machine (also known as a Wurster machine, after inventor DaleWurster). Several examples of the Wurster coating machine are disclosedin U.S. Pat. Nos. 3,196,827,3,110,626, 3,880,116, 4,330,502, 4,535,006and 5,236,503.

The Wurster coating machine is typically used to layer, coat orencapsulate lightweight powders, particles, granules or pellets of solidmaterials, including pharmaceutical drugs. Often, coatings are appliedto modify the release of the substrate (protective barrier, tastemasking, enteric coating, delayed release or sustained release). Apredetermined quantity of these coated particles are usually packagedwithin an edible gelatin capsule or compressed into a tablet. Thedistribution uniformity of the applied substance may not be criticalbecause the capsule or tablet contains multiple units and the averagecoating thickness of all of the pellets within the capsule dictate theaverage release properties and performance of the overall dosage form.

As described below, the Wurster machine generates an upward stream ofair or other gases such as nitrogen to circulate a substrate (particles,pellets, powders, etc.) through a vertical spray of coating liquidwithin a product container. As the substrate cycles through the spray aminute amount of coating material is deposited on its surface. Thenumber of cycles the substrate completes determines the thickness of thefinal coating layer.

The conventional Wurster machine works well when the particles are fineand lightweight (such as grains of powder). However, due to flow-relatedproblems inherent in the design, the conventional Wurster machine failsto provide a uniform distribution of coating on heavier tablets becausethe heavier tablets do not uniformly cycle through the machine. TheWurster-coated tablets cannot be used for applications which requireuniform, predictable and consistent distribution coatings on all tabletswithin a particular batch.

Certain types of pharmaceutical controlled-released tablets requirehigh-precision coatings because the thickness of the coating governs thetime of release and the release rate of the active ingredient of thetablet and thereby directly influences the effectiveness of themedication. The conventional Wurster machine is incapable of providing ahigh-precision coating on tablets, in part due to the followingflow-related problems, each of which adversely effects the precision ofthe coating of each tablet or particle in the batch.

The conventional Wurster machine also creates undesirable turbulence andintroduces high shear forces to the substrate as it cycles through themachine. The fine and lightweight substrates typically used withconventional Wurster machines are not adversely affected by the violenttraumatic forces they must endure during each cycle. However, when aconventional Wurster machine is used to coat heavier tablets, the highshear forces generated during each coating cycle are capable of damagingthe tablets and the resulting attrition rate of the tablets isunacceptable.

The heavier tablets are also more difficult to introduce into the highvelocity airstream of the Wurster machine, usually causing some of thetablets to accelerate directly into hard structures within the machine,such as a nozzle assembly. The impact can easily shatter or otherwisedamage the tablets.

Once a substrate is processed using the Wurster machine, the substratemust be removed from the product container. This is conventionallyaccomplished through a pivotal bottom door which, when opened, allowsthe coated substrate to simply fall by gravity into an awaiting andsuitable container. Although this emptying process is effective, theprocess exposes both the substrate and the interior of the productcontainer to the environment. Not only does this exposure introduceundesirable contamination to the product container, it also subjects theoperators of the machine unnecessarily to potentially hazardousmaterials. To this end, it would be beneficial to remove the coatedsubstrate from the coating machine using a more controlled andpredictable process without undue complexity and without affecting themachine's operation.

Another problem with Wurster machines is that they are relativelydifficult to clean. The cleaning procedure typically requires theopening of the lower end of the product container and the application ofan appropriate cleaning fluid. Some coating machines have spray nozzleswithin the coating machine to initially wash out any residual materialdeposited along the interior surfaces of the machine after the coatingprocess. The cleaning fluid from these nozzles washes the interiorsurfaces of the machine and typically drains through the open lower end.Sometimes, however, the material being processed within the productcontainer comprises a drug or other material which may be hazardous ifaccidentally inhaled, swallowed or even touched by personnel assigned tooperate and clean the coating machine. It would therefore be beneficialto ensure that a maximum amount of this potentially hazardous residue iswashed from the expansion chamber and the product container whileisolating the contaminated waste from the surrounding environment.(i.e., without having to opening the machine).

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a fluidized-bed typecoating machine which overcomes the deficiencies of the prior art.

It is another object of the present invention to provide a Wurster-typecoating machine which encourages even and predictable flow of tabletslocated in the down-bed.

It is another object of the present invention to provide a Wurster-typeparticle-coating machine which encourages tablets to flow radiallyinwardly along a distribution plate between the down-bed and an up-bed.

It is another object of the invention to provide a Wurster-typeparticle-coating machine which includes a central nozzle assemblylocated at the distribution plate for discharging a spray of coatingliquid and which further includes structure to redirect tablets from thedown-bed to the up-bed without impacting the central nozzle assembly.

It is another object of the invention to provide a Wurster-typeparticle-coating machine which is particularly suited to accurately coatheavier particles such as tablets.

It is another object of the present invention to provide a Wurster-typeparticle-coating machine which includes a partition which is shaped toprovide an atraumatic transition of the tablets moving from the down-bedinto the up-bed.

It is another object of the present invention to provide a Wurster-typeparticle-coating machine which cycles the tablets within the machinebetween the down-bed and the up-bed in a smooth, efficient, andconsistent manner so that the resulting coating distribution of eachtablet is consistent and predictable and tablet-attrition is minimized.

It is another object of the present invention to provide a Wurster-typeparticle-coating machine which permits discharge of the substrate(particles or tablets) through an opening at the center of the orificeplate at the base of the insert when multiple partitions are used.

It is another object of the present invention to provide a Wurster-typeparticle-coating machine which is easy to operate during the coatingprocess and facilitates cleaning without disassembly of the Wursterinsert.

SUMMARY OF THE INVENTION

The foregoing objects of the invention are met through variousimprovements to a Wurster-type fluidized bed apparatus for applying acoating liquid onto the surface of particles. The coating liquid isgenerally comprised of substances in a solution, suspension ordispersion in water or organic solvent (in some cases a molten liquidmay be used). The apparatus includes a vertically disposed cylindricalor slightly conical product container having a peripheral wall, at leastone cylindrical partition defining a centrally located up bed region anda peripherally located down bed region. The product container furtherincludes an upper end connected to an expansion chamber and a lower endincluding an orifice plate having a plurality of openings for passage offluidized air. A nozzle is centrally located through the orifice plateand is adapted to generate a spray of coating liquid upwardly into theup bed. Particles located within the product container circulateupwardly through the partition and the coating liquid spray, between theup bed and the down bed.

A feature of the invention comprises a nozzle ramp which is placedaround the nozzle. The nozzle ramp is generally cusp shaped and includesa nozzle-ramp surface. The nozzle ramp is centrally positioned aroundthe nozzle, and may be used within the product container with or withouta nozzle sleeve. The nozzle ramp is directed upwardly towards thepartition so that the nozzle-ramp surface directs particles movinggenerally horizontally across the orifice plate from the down bedupwardly into the partition and the up bed. The nozzle ramp may furtherinclude air passages for passing fluidized air upwardly to thenozzle-ramp surface. The air flow through these perforations provides acushion of air at the surface of the nozzle ramp which minimizes theimpact of tablets against this structure. One purpose of the perforatednozzle ramp is to atraumatically guide the horizontally flowing tabletsvertically and upwardly into the up bed inside the partition.

A third feature of the invention comprises a nozzle ramp which is placedaround the nozzle. The nozzle ramp is generally cusp shaped and includesa nozzle-ramp surface. The nozzle ramp is centrally positioned aroundthe nozzle and is directed upwardly towards the partition so that thenozzle-ramp surface directs particles moving generally horizontallyacross the orifice plate from the down bed upwardly into the partitionand the up bed. The nozzle ramp may further include air passages forpassing fluidized air upwardly to the nozzle-ramp surface. The air flowthrough these perforations provides a cushion of air at the surface ofthe nozzle ramp which minimizes the impact of tablets against thisstructure. The purpose of the perforated nozzle ramp is toatraumatically guide the horizontally flowing tablets vertically andupwardly into the up bed inside the partition.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side view schematic (labeled PRIOR ART) of a priorart Wurster-type bottom spray particle-coating machine showing a productcontainer, a partition, a nozzle, a down-bed, an up-bed, a nozzlesleeve, and a plurality of tablets or pills (represented by spheres)being coated as they circulate through the machine;

FIG. 2 is an enlarged sectional view (labeled PRIOR ART) of a portion ofthe prior art Wurster-type coating machine of FIG. 1 (shown without thenumerous particles, for clarity) including particle-flow arrowsrepresenting the flow of particles during their transition from thedown-bed into the up-bed air-stream, and further including a dashed linerepresenting a “dead-zone” within the product container;

FIG. 3 is a sectional side view schematic of an improved Wurster-typebottom spray particle-coating machine showing a product container, apartition having a peripheral skirt, a nozzle, a down-bed, an up-bed, anozzle sleeve, a truncated cusp-shaped nozzle-ramp, and a plurality oftablets or pills (represented by spheres) being coated as they circulatethrough the machine, according to the present invention;

FIG. 4 is an enlarged sectional view of a portion of the improvedWurster-type coating machine of FIG. 3 (shown with only selectedparticles, for clarity) showing details of the peripheral skirt, thenozzle-ramp, the nozzle sleeve, and the peripheral air-injection systemand further including particle-flow arrows representing the flow of theselected particles during their transition from the down-bed into theup-bed air-stream, according to the invention;

FIG. 5 is an enlarged sectional view of a portion of the improvedWurster-type coating machine of FIG. 3, showing details of a dead-zone(shown in dashed line), the peripheral air-injection system, and flowarrows, according to the present invention;

FIG. 6 is an enlarged section view of a portion of the improvedWurster-type coating machine of FIG. 3 showing details of thenozzle-ramp and nozzle sleeve (showing a conventional straight-edgedpartition), and including flow arrows, according to the presentinvention;

FIG. 7 is a plan view of the peripheral air-injection system showing aninlet conduit, a ring manifold assembly including a plurality ofinwardly-directed discharge outlets, according to the invention;

FIG. 8 is a sectional view of the peripheral air-injection system ofFIG. 7, taken along the lines 8—8 of FIG. 7, according to the presentinvention.

FIG. 9 is a side view of the nozzle sleeve according to the invention

FIG. 10 is a cross-sectional side view of the nozzle sleeve of FIG. 9,according to the invention;

FIG. 11 is a partial-sectional side view of the nozzle-ramp, accordingto the invention;

FIG. 12 is a bottom view of the nozzle-ramp showing details of thethrough passages, according to the invention;

FIG. 13 is a sectional assembly view of the partition skirt showingupper and lower sections, taken along the lines 13—13 of FIG. 14,according to the invention;

FIG. 14 is a bottom view of the partition skirt, according to theinvention;

FIG. 15 is a partial-sectional side view of the partition skirt,according to the invention;

FIG. 16 is a plan view of a conventional orifice plate;

FIG. 17 is a sectional side view of the conventional orifice plate ofFIG. 16, taken along the line 17—17 of FIG. 16;

FIG. 18 is a sectional view of a multi-partition coating machine havinga central discharge assembly, according to another feature of thepresent invention, taken along the line 18—18 of FIG. 19;

FIG. 19 is a bottom view of the multi-partition coating machine of FIG.18, according to the invention, showing details of the nozzle assembliesand the central discharge;

FIG. 20 is an enlarged sectional side view of the central dischargeassembly of FIG. 18, according to the invention, shown in a closedposition;

FIG. 21 is the central discharge assembly of FIG. 20, according to theinvention, shown in an open position; and

FIG. 22 is a sectional side view schematic of the improved Wurster-typebottom spray particle-coating machine similar to the one shown in FIG.3, and further including a split lower plenum which divides the air orgas stream and provides independent control of the flow of air or gas inthe up and down bed regions of the Wurster machine, according to thepresent invention.

DETAILED DESCRIPTION OF THE FEATURES OF THE INVENTION

By way of background and introduction, the present invention providesimprovements relating to tablet flow and handling during the coatingprocess of an otherwise conventional Wurster-type bottom-spray particlecoating machine (hereinafter referred to as “Wurster machine”). Tobetter understand and appreciate the improvements of the presentinvention, a detailed description of the structure and operation of aconventional Wurster machine is first provided.

A. Description of a Conventional Wurster Machine

Referring to FIGS. 1 and 2 (labeled PRIOR ART), a conventional Wurstermachine 10 is shown, including a generally conical product container 12,an orifice plate 14, a lower plenum 16, a central nozzle 18 projectingupwardly through a central opening 20 located within orifice plate 14, anozzle sleeve 22, and a cylindrical partition 24. The product container12 may be cylindrical or conical in shape and is mounted to an uppersurface 26 of orifice plate 14, while the lower plenum 16 connects witha lower surface 28 of orifice plate 14. The upper end of the productcontainer 12 is connected to an expansion chamber (not shown).

As shown in FIGS. 16 and 17, orifice plate 14 includes a plurality oforifices 30 which are arranged in such a manner as to allow air to flow(represented in FIGS. 1 and 2 by arrows 15) from the lower plenum 16through the orifices 30 and into the product container 12, to create afluidized bed, as is described in greater detail below. Orifice plate 14typically includes two sections of relative porosity concentricallypositioned around the central opening 20; an up bed section 14A (alsoreferred to as an “up bed plate”), and a down bed section 14B (alsoreferred to as a “down bed plate”). Air flow from the lower plenum 16will behave differently in the up bed plate and the down bed plate. Theair flow generated by a remote blower unit (not shown) is restricted ina controlled and predictable manner so that a desired fluidity of thesolid particles located within the product container is maintained. Theup bed section 14A of the orifice plate (located under the partition 24and adjacent to the nozzle 18) includes a large number of orifices 30which allow a high volume of air from the lower plenum 16 to passthrough the orifice plate 14 and up through the partition 24 at arelatively high velocity. This upward flow of high velocity air iscalled the up bed (represented by arrows 33 in FIGS. 1 and 2) and isused to pneumatically transport the substrate vertically past the spraynozzle during each cycle.

The ring-shaped region outside of the partition 24 within the productcontainer 12 is referred to as the down bed (represented by arrows 37 inFIGS. 1 and 2). The down bed section 14B of the orifice plate 14 has aporosity (number, diameter and distribution of orifices 30) which allowssufficient air flow to penetrate the substrate from the lower plenum 16and to maintain the substrate located in the down bed in near-weightlesssuspension. This influx of air causes the particles 27 to behavesomewhat like a fluid and travel rapidly and freely downward in the downbed 37 and subsequently be drawn horizontally through the transition gapand redirected upwardly into the up bed. The amount of air flow requiredto produce the near-weightless suspension of the substrate depends onthe size and shape of the particular substrate. In general, tabletsrequire significantly more air to produce this condition than pellets orfine particles and the orifice plate 14 must be designed accordingly.

The central nozzle 18 of the prior art Wurster machine 10 in FIG. 1 ismounted through the central opening 20 of the orifice plate 14 so thatits discharge outlet is directed upwardly into the partition 24 andproduct container 12. The nozzle 18 ejects a highly atomized spray ofcoating liquid into the partition, creating a “coating zone” 32 which isusually shaped as a narrow volumetric ellipse. The exact shape of thecoating zone may be controlled according to the size of the substratebeing coated and the pattern density in the partition. The atomizedspray is discharged from the nozzle at about 300 meters per second.

Centrally and vertically mounted within the product container is thecylindrical partition 24 whose diameter is about half the diameter ofthe product container 12 (as measured at the base of the productcontainer). The cylindrical partition 24 is positioned within theproduct container 12 adjacent to and above the central nozzle 18 so thatthe spray pattern discharged from the nozzle 18 extends into thepartition 24. As is understood by those skilled in the art, thepartition 24 is used to help direct particles 27 located within theproduct container 12 as they circulate throughout the Wurster machine,and further to protect the integrity of the spray pattern 32. Thepartition 24 also helps separate particles 27 moving upwardly in acentrally located up bed 33 through the coating zone 32 from particles27 that are falling back towards the orifice plate 14 in a peripherallyoriented down bed 37 with respect to the product container 12.

The partition 24 includes a lower edge or rile 25 and is positionedwithin the product container 12 so that the lower rim 25 is located apredetermined distance from the upper surface 26 of the orifice plate14. The space (or “transition gap”) that is created between the lowerrim 25 of the partition 24 and the upper surface 26 of the orifice plate14 forms a “transition zone” where particles 27 located in theperipheral down bed 37 are radially-inwardly drawn through adifferential in pressure back into the central up-bed 33.

Conventional Wurster machines do not shield the relatively hard nozzleassembly 18 and will typically result in particle attrition and breakageas the particles impact the nozzle assembly 18 as they accelerate fromthe down bed 37 to the up bed 33.

One improvement over the conventional Wurster machine regarding theproblem of particle attrition and breakage due to an exposed nozzleassembly 18 is disclosed in commonly owned U.S. Pat. No. 5,236,503. Thispatent discloses a protective nozzle sleeve 22 which is a tubepositioned around the nozzle assembly 18. The nozzle sleeve 22 disclosedin U.S. Pat. No. 5,236,503 (and shown in FIG. 1 of the present patentapplication) prevents particles 27 from entering the spray pattern untilit is more fully developed, thereby increasing productivity.Furthermore, it keeps the substrate from encountering the extremely highcompressed air velocity at the tip of the spray nozzle thereby reducingparticle attrition and breakage. Unfortunately, some of the particlesaccelerating into the up bed 33 in the prior art Wurster machine (withthe improved protective nozzle sleeve 22) still impact the nozzle sleeve22, resulting in a measurable attrition rate. Such impact damage to theparticles 27 will only increase as the mass of the particles increases,such as when relatively heavy tablets are cycled through the coatingmachine.

Another problem with the prior art nozzle sleeve 22, as shown in FIG. 1of the present patent application and also disclosed in U.S. Pat. No.5,236,503 is that the sleeve itself is prone to trapping particles 27 atthe completion of the process, when fluidization air flow is stopped.Once trapped within the hollow nozzle sleeve 22, the particles 27 willno longer circulate within the machine and may further interfere withthe air flow around the nozzle, causing its spray pattern to be alteredand resulting in undesirable agglomeration of the particles. Thesetrapped particles must be manually removed prior to loading a subsequentbatch.

B. Conventional Wurster Operation

In operation of the prior art Wurster machine shown in FIGS. 1 and 2, asupply of particles 27 that are to be coated are placed within theproduct container 12. The size of the particles 27 are typically between100 and 2000 microns in diameter. The particular operating parameters(such as the specific size, permeability, and arrangement of theorifices 30 in orifice plate 14, and the height of the transition gap)of the Wurster machine will vary depending on the size and type ofparticles 27 being coated.

When the particles 27 are positioned within the product container 12, aflow of filtered air is drawn from an air handling unit (not shown), theproduct container itself, and an expansion chamber (not shown) using anappropriate blower fan (also not shown) creating a negative pressurewithin the expansion chamber and the product container 12. This negativepressure causes air (arrows 15) to be drawn upwardly through orifices 30of orifice plate 14 from lower plenum 16. As the air is drawn into theproduct container 12 through orifices 30, it passes through both the upbed 33 and the down bed 37 (moving upwardly) penetrating and influencingthe particles 27 located in the down bed 37, as shown in FIG. 1. Theupward flow of air causes each particle 27 of the down bed 37 toeffectively float or become suspended on a cushion of air as the airflow finds its way upward into the product container 12 to try toequilibrate the negative pressure in the expansion chamber. The“floating” particles 27 become “fluidized”, behaving more like a fluidthan a mass of solid particles. Air (arrows 15) from plenum 16 is alsodrawn into the product container 12 through opening 20 and into nozzlesleeve 22 and further up into partition 24 adding to the up bed airstream 33.

After the flow of air from the lower plenum 16 fluidizes the particles27 within the product container 12, the central nozzle 18 is activatedto discharge a controlled spray pattern 32 of coating liquid upwardlyinto partition 24, as shown in FIGS. 1 and 2. The spray liquid generallyis comprised of a solution, suspension or dispersion in water or organicsolvent (in some cases a molten liquid may be used) and is ejectedupwardly from the nozzle 18 at a high velocity around 300 meters/second,depending on the particular type and size of particle being coated, thetype of coating material used, and the desired coating characteristics.

As the particles 27 rise rapidly upward in the high-velocity up bed airstream 33 created by the nozzle 18, they contact micro-atomized dropletsof the coating liquid and become coated before slowing down within theexpansion chamber (not shown). As the particles 27 continue to rise inthe partition 24 and into the expansion chamber, excess moisture fromthe applied coating liquid evaporates

The high-velocity air stream spouting from the upper end of thepartition 24 forces the particles 27 radially outwardly in the expansionchamber (as represented in FIG. 1 by arrows 35). Once away from theupstream lift provided by the up bed 33, the particles 27 are influencedby gravity and fall within the product container 12 in the down bed 37,eventually reaching the orifice plate 14.

The high volume and velocity of the airflow into and through thepartition 24, combined with the high velocity of the air from the nozzle18, generates a very strong negative pressure in the transition zonelying adjacent to the nozzle 18 and the orifice plate 14 relative to themeasured pressure within down bed 37. This creates a pressuredifferential. The pressure differential draws the particles 27 that arelocated in the peripheral down bed 37 radially inwardly through thetransition zone and into the up bed 33. The up bed again accelerates theparticles 27 up into the partition 24 and through the coating zone 32.

The cycle is repeated for all particles 27 located within the productcontainer 12, until a desired coating thickness is formed on allparticles of the batch.

C. Problems with the Conventional Wurster

As discussed in the Background section of this specification, theabove-described Wurster machine is generally effective at coating fineparticles within the product container. The longer the particles arekept circulating through the spray of the coating liquid, the greaterthe thickness of the coating on each particle, and the greater theconsistency between coated particles of the same batch. The Wurstermachine, however, fails to provide an accurate and predictabledistribution of coating on the particles as the particles increase inmass and size (such as when tablets or pills are cycled through themachine). The conventional Wurster machine includes three mainparticle-flow-related problems which are inherent in its design and areillustrated in FIG. 2 and described below.

A first flow-related problem of the Wurster machine relates to an unevencirculation flow of particles 27 located in the down bed 37 resulting inthe creation of a peripheral “dead zone” 40 (shown in dashed lines inFIG. 2). During the coating process, the heavy tablets (or particles 27)within the down bed 37 exert pressure on the orifice plate 14,particularly along the peripheral wall of the product container 12.Owing in part to this “loading”, a region of low-flow (and in somecases, no flow) is created along the outer perimeter of the productcontainer 12 within an outer and lower section of the down bed 37. Thedead zone 40 generally extends approximately 50 mm above the orificeplate 14 and about 50 mm out into the product container.

The tablets (or particles 27) located within this peripheral dead zone40 of the down bed 37 tend to slow down and even stop relative totablets (or particles 27) located radially inwardly within the down bed37. These slower moving tablets (or particles 27) in the dead zone 40fail to circulate as often as the other tablets and will therefore havean adverse effect on the consistency of coating distribution betweentablets within the same batch. During the coating process, the surfaceproperties of the tablets change, and flow behavior in most instancesworsens (the flow of tablets slows down). This decrease in tablet flowtends to increase the probability that a slow or dead zone will formalong the perimeter of the base of the product container.

Again, referring to FIG. 2, a second flow-related problem of the priorart Wurster machine 10 relates to a misdirected flow of particles 27. Asa pressure differential is created in the transition zone between thedown bed 37 and the up bed 33 of the Wurster machine, some of theheavier particles 27 (e.g., tablets) lying close to the orifice plate 14fail to divert upwardly into the airstream of the up bed 33 and actuallyimpact against either nozzle 18, or nozzle sleeve 22 (if one is used).This impact path is represented by an arrow 42 in FIG. 2 and willinvariably increase attrition and breakage of the tablets.

A third flow-related problem of the prior art Wurster machine 10 is thata percentage of tablets 27 located within the down bed and adjacent tothe partition 24 are violently and traumatically pulled into the up-bed33 by a pressure differential, resulting in tablet attrition andbreakage. This traumatic flow path is represented by an arrow 44 in FIG.2.

Another problem associated with the prior art Wurster machine becomesapparent after the coating process is complete and particles 27 must beremoved from the product container 12. The conventional process includeshinging open the lower plenum 16 (and the orifice plate 14) from thelower portion of the product container 12 and literally dumping thecoated particles 27 from the product container 12 into an awaitingcontainer. Not only does this crude emptying procedure expose theimmediate environment (including workers) to potentially hazardousmaterials (such as drug residue), it also exposes the freshly coatedparticles and the interior surfaces of the product container and orificeplate to possible contamination.

Also, the conventional Wurster machine is difficult to clean, usuallyrequiring hinging open the lower plenum 16 (as described above) andspraying a cleaning solution throughout the product container andexpansion chamber, allowing the waste cleaning fluid (which iscontaminated and potentially hazardous) to pour from the machine throughthe open lower plenum 16. As during the above-described particleemptying procedure, this prior art cleaning process introducespotentially hazardous materials to the immediate environment which aredifficult to handle and contain.

Finally, the prior art nozzle sleeve 22, described in commonly ownedU.S. Pat. No. 5,236,503 fails to adequately prevent particles impactingits surface, and further is prone to accumulating and trapping particlesat the completion of the coating process. U.S. Pat. No. 5,236,503 ishereby incorporated by reference into this specification.

D. Description of the Present Invention

Referring now to FIG. 3, a Wurster machine is disclosed includingimproved features according to the present invention. The improvedfeatures solve the above-discussed problems of the conventional Wurstermachine so that the improved Wurster machine may be used to accuratelyand efficiently coat heavier particles 27, such as tablets and pills(hereinafter referred to as “tablets” 27).

1. Air-Injection Manifold

As discussed above in the background section of the specification, oneproblem inherent in the design of the conventional Wurster machine isthe existence of dead zones, wherein tablets 27 become stagnant andcycle fewer times than other tablets in the same batch. According to afirst feature of the invention, as shown in FIGS. 3, 4, 5, 7 and 8, anair-injection manifold 50 is provided around the lower end of theproduct container 12 adjacent to the orifice plate 14 and whichovercomes the problems associated with the creation of dead zones inprior art coating machines.

As shown in FIGS. 5, 7 and 8, ring manifold 50 is circular and includesan inner surface 52 having a lower edge 54, an upper surface 56 having acircumferential channel 58 located immediately adjacent to the innersurface 52, a bottom surface 59, and a plurality of openings 60 evenlyspaced along the inner surface 52 immediately adjacent to the lower edge54. Openings 60 are directed radially inwardly towards the center of thecircular manifold 50. Each of the openings is in fluid communicationwith an internal circumferential conduit 62 which is shown in section inFIGS. 3, 4, 5, and 8. An inlet conduit 64 (FIG. 7) connects with theinternal conduit 62 so that air supplied under pressure to inlet conduit64 flows within conduit 62 and discharges evenly throughout theplurality of openings 60 around the inner surface 52. This discharge ofair flow from openings 60 results in a radially directed flow of air(airflow from selected openings 60 is represented by arrows 65 in FIG.7).

Since manifold 50 is intended to be used in a clean environment, it ispreferably made as an assembly of parts which may be selectivelydisassembled to access and clean all surfaces. To this end, internalconduit 62 is preferably formed by fitting a conduit ring 66 and abottom sealing ring 68 with an outer main ring 70, as shown in FIGS. 3,4, and 8.

According to the invention, manifold 50 is positioned between a sidewall 72 of product container 12 and orifice plate 14, as shown in FIGS.3, 4, and 5, and is sized and shaped so that a lower end 74 of side wall72 snugly fits within the circumferential channel 58 and forms a smoothtransition between an inner surface 75 of side wall 72 and inner surface52 of manifold 50. Inner surface 52 of ring manifold 50 preferablyangularly aligns with the conical product container 12. Bottom surface59 of manifold 50 is mounted to the upper surface 26 of orifice plate 14so that the openings 60 of manifold 50 lie immediately adjacent uppersurface 26 of orifice plate 14. With this arrangement, according to theinvention, air (or any fluid, including cleaning liquids or rinse water)that is introduced under pressure into inlet conduit 64 will dischargethrough openings 60 in a radially inward direction across the uppersurface 26 of orifice plate 14. As illustrated in FIG. 5, this inwardlydirected blast of air 65 from manifold 50 effectively prevents the deadzone 40 from forming by forcing all tablets 27 located in this region tomove horizontally towards the transition zone and nozzle 18 (asrepresented by arrows 61 in FIG. 5). Tablets 27 are not shown in deadzone 40 of FIG. 5 for clarity so that air flow arrow 65 can be seen andunderstood. According to the invention, manifold 50 keeps all of theheavy tablets moving evenly from down bed to up bed so that an otherwiseconventional Wurster machine may be used to coat heavy tablets 27without creating a peripheral dead zone 40 within the product container12. Additionally, as discussed in greater detail below, the volume andvelocity of the air from openings 60 may be adjusted to compensate forchanging tablet surface flow properties during the application ofcoating material to the tablets. This manifold air adjustability isindependent of the process air flow through the orifice plate in the upbed 33 and down bed 37 regions.

The effectiveness of introducing a radially directed blast of air fromthe periphery of the product container 12 along the upper surface 26 ofthe orifice plate 14 can be appreciated through the illustrations ofFIGS. 4 and 5. Arrow 65 of FIG. 5 represents the force of the radiallyinwardly directed blast of air while arrow 78 represents the “loading”or force exerted by the tablets 27 located in the down bed 37 on orificeplate 14. The horizontal force 65 generated by the discharged air fromopenings 60 of manifold 50 move the lower tablets 27 of the down bed 37radially inwardly as shown by arrow 61, to effectively make room forother tablets 27 of down bed 37 and to keep all of the tablets 27 movingin a smooth and consistent flow from down bed 37 to up bed 33.

Referring to FIG. 4, a representative tablet 27 moves on a radiallyinward path (arrow 65) in response to the radially inwardly directedflow of air discharged by openings 60 of manifold 50. The horizontal airinjection flow created by manifold 50 cooperates with the conventionalflow of air passing through openings 30 of orifice plate 14 to maintainfluidization of tablets 27.

The air pressure used to feed manifold 50 will vary depending on thesize and shape of the tablets 27 being coated, their changing flowproperties during a coating process, the size and shape of theparticular product container 12, the particular configuration of theorifice plate 14, and other operational and structural parameters of themachine. The air pressure measured in one operational example was about20 pounds per square inch (p.s.i.).

According to another aspect of the invention, the supplied air pressuremay be controlled so that the velocity of the air discharged fromopenings 60 will vary at predetermined time periods during a coatingprocedure. Tablets 27 are typically provided with a lubricant on theirsurface which allows them to flow easily throughout the coating machineduring the first few minutes of the coating process. As the coating isapplied, however, the surface of each tablet 27 tends to become a bittacky, resulting in a slower descent rate in the down bed 37. Bycontrolling the radially directed air injection (independent of the flowof air through the orifice plate) over time, the flow resistance causedby the “tackiness” of tablets 27 in the down bed 37 can be accounted forand minimized, resulting in a consistent down bed (and up bed) behavior.For most instances, during the coating process, the air flow throughoutlets 60 of manifold 50 may be controlled to gradually increase invelocity and volume.

2. Nozzle Sleeve

Referring to FIGS. 3, 4, 9, and 10, a nozzle sleeve 90 is shownaccording to a second feature of the present invention. Nozzle sleeve 90is hollow and includes a generally cylindrical base portion 92, having acircular bottom edge 94, a truncated conical upper portion 96 having acircular upper edge 98 (defining an upper opening 99), and a centralhollow passage 100. Nozzle sleeve 90 is sized and shaped to fit aroundnozzle 18 (see FIGS. 3 and 6). Passage 100 and the diameter of upperopening 99 may be sized with respect to the diameter of nozzle 18 sothat air may flow freely up through passage 100 and through upperopening 99, adjacent to nozzle 18 during the operation of the machine,as described below. By directing air through the passage 100 of nozzlesleeve 90 in this manner, the discharge of air through upper opening 99adjacent to nozzle 18 may assist in shaping and controlling the shapeand characteristics of the spray pattern generated by the nozzle.Alternatively, the upper opening 99 may be sized to tightly receivenozzle 18 so that no air (or minimal air) will pass through nozzlesleeve 90 during the operation of the machine. Nozzle sleeve 90 issecured in position with bottom edge 94 abutting against orifice plate14.

Once in position around nozzle 18 within product container 12, andduring the coating operation, nozzle sleeve 90 serves three functions.First, nozzle sleeve 90 protects tablets 27 from directly impacting theharder surfaces of nozzle 18 during operation, as described below.Second, conical upper portion 96 is shaped to accommodate the naturalflow of tablets 27 as they are drawn into the up bed 33 from the downbed 37, as shown in FIG. 3. Third, the hollow passage 100 and theconical upper portion 96 direct air from the lower plenum 16 to assistin shaping the up bed 33 and the coating zone 32.

Nozzle sleeve 90 is preferably made from strong, somewhat resilientplastic, such as a Delrin, or an appropriate rubber, such as silicone,and is preferably adapted to be easily installed within a coatingmachine and quickly and easily replaced to minimize setup time. Theparticular dimensions and shape of nozzle sleeve 90 may vary accordingto particular parameters of the coating machine, as is understood bythose skilled in the art. The nozzle sleeve may be separate orintegrated into the nozzle ramp, and may be solid or perforated topermit air flow in proximity to the spray nozzle.

3. Nozzle Ramp

As mentioned above, a problem with the conventional Wurster coatingmachine is that heavier tablets 27 are traumatized during theirtransition from the down-bed 37 of the product container 12 to thecentral up-bed 33 through the partition 24. The heavier tablets used ina conventional Wurster machine may also be damaged by impacting thenozzle assembly during bed transition. The up bed 33 moves much fasterthan the peripheral down bed 37 within the product container and astrong negative pressure is developed around the center of the orificeplate and within part of the partition. As discussed above, thisnegative pressure rapidly draws tablets 27 from the peripheral down bed37 radially inwardly along a horizontal path into the airstream of theup bed 33. Owing to the mass of the tablets 27, the horizontal componentof the inertia imparted to the tablets 27 by the negative pressure isoften too great for the upwardly moving airstream of the up bed 33 tocompletely vertically redirect the horizontally moving tablets 27 beforesome of the tablets 27 impact the centrally located nozzle assembly 18,nozzle sleeve 90 (if one is used) and/or other tablets 27 entering fromopposing directions along the orifice plate 14.

Referring to FIGS. 4, 6, 11, and 12, a nozzle ramp 102 is shown,according to a second feature of the present invention, which overcomesthe above-mentioned problem of tablets 27 impacting nozzle 18 as theyenter into the up bed 33. Nozzle ramp 102 includes a circular base 104having a perimeter 106, a hollow cylindrical center 108 having a sidewall 110 and a top edge 112, and an arcuate ramp surface 114 (having ashape that is similar to a cusp) positioned between perimeter 106 ofbase 104 and top edge 112.

As shown in FIG. 3, nozzle ramp 102 is centrally positioned withinproduct container 12 with its base 104 mounted flush against the uppersurface of orifice plate 14 (using bolts, for example). Nozzle ramp 102may be used within the product container with or without a nozzlesleeve. Should a nozzle sleeve be used, hollow center 108 is preferablysized and shaped to accommodate both nozzle 18 and nozzle sleeve 90,described above, or alternatively, a conventionally shaped nozzle sleeve22, such as the one shown and described in U.S. Pat. No. 5,236,503.Further, nozzle ramp 102 may be formed with an integral nozzle sleeve(not shown), however, it is preferred that nozzle sleeve remain as aseparate and attachable part so that tablet and air flow characteristicscan be better controlled.

Nozzle ramp 102 further includes a plurality of vertically disposedpassages 116 which are preferably arranged in concentric rings passingbetween the arcuate ramp surface 114 and circular base 104, as shown inFIGS. 6 and 11 (in section). These vertical passages 116 are sized andshaped to exactly align with corresponding openings 30 of a conventionalorifice plate 14, which is shown in FIG. 6, so that air passing throughopenings 30 from lower plenum 16 freely passes through the alignedvertical passages 116 of nozzle ramp 102 and becomes discharged at anupper end of each respective passage 116 along arcuate ramp surface 114.Passages 116 are preferably either equal to or larger than thecorresponding openings 30 of orifice plate 14. Orifice openings 30 aresized to control the flow of air within passages 116.

The purpose of the nozzle ramp 102 is to direct air from the lowerplenum 16 along a curved circular ramp surface so that the fast moving,horizontally driven tablets 27 can be atraumatically coerced to follow avertical trajectory using a cushion of air. As discussed below, the useof nozzle ramp 102 will minimize undesirable impacting of tablets 27against nozzle 18 or nozzle sleeve 90. The conical upper portion 96 ofnozzle sleeve 90, according to the above described feature of thepresent invention, preferably generally aligns with arcuate ramp surface114 of nozzle ramp 102, as shown in FIGS. 3 and 4 so that tablets 27being diverted to the up bed by the nozzle ramp 102 may follow a lesssevere arcing path (as shown as arrow 118 in FIG. 4) and still avoidimpacting any portion of nozzle sleeve 90.

4. Partition Skirt

The tablets 27 located in the down bed 37 (FIG. 1) of a Wurster machinegenerally move downwardly at a rate of one meter in approximately 10 to30 seconds. When these tablets 27 are drawn into the up bed 33, wherethey accelerate to approximately 5 to 10 meters per second, theyencounter an atomizing air velocity of about 300 meters per second. Thisviolent change in velocity causes great transitional trauma and highshear to the relatively fragile tablets 27 and will likely increase theattrition rate of the tablets of the batch early in the coating process.The relatively sharp lower edge 25 of the conventional partition 24which separates the down bed 37 and the up bed 33 only exacerbates thetransitional trauma to the tablets 27 as they are drawn into the fastmoving upward current of the up bed.

Referring to FIGS. 3, 4, 13, 14, and 15, a partition skirt 120 accordingto another feature of the present invention is provided at the loweredge 25 of partition 24 which overcomes the problems relating totransitional-trauma of the tablets 27 entering the high velocity up bed33. Skirt 120 is a ring-shaped cone having a angled outer surface 122 atangle A (FIG. 13), a cylindrical inner bore 124, and a lower surface126. Bore 124 is sized and shaped to snugly receive partition 24 so thatskirt 120 may be secured to the lower end of partition 24 (preferably ina manner that allows skirt 120 to be quickly and easily removed frompartition 24 if necessary). The diameter of bore 124 is preferably equalto or slightly larger than the outside diameter of partition 24.

Lower surface 126 of skirt 120 is preferably beveled at a prescribedangle B, as shown in FIG. 13, forming an inwardly directed funnel shapewhich extends between angled surface 122 and bore 124. Lower surface 126functions as an “on-ramp” allowing tablets 27 adjacent to skirt 120 to“get up to speed” before entering the high velocity up bed 33. The exactdegree of angle B of the lower surface 126 will vary depending on thesize, shape and weight of the tablets 27, the dimensions of the productcontainer 12, coating characteristics, as well as other structural andoperational factors and parameters, but angle B will generally be in therange of 15 and 30 degrees. Lower surface 126 preferably includes arounded outer edge 129, as shown in FIG. 15.

Tablets 27 located in down bed 37, in particular adjacent to thepartition 24, will be directed away from partition 24 as they descenddown bed 37 by angled surface 122 until they reach lower surface 126 atwhich point tablets 27 will gradually pick up speed due to the pressuredifferential created by the up bed 33 and move inwardly along lowersurface 126. As tablets 27 move inwardly along lower surface 126, theywill gradually accelerate before “falling upward” into the high velocityup bed 33. Lower surface 126 allows tablets 27 to gain speed and therebyreduces the effects of shear and other mechanical stresses created bythe gradient between down bed 37 and up bed 33. This less traumaticintroductory path into up bed 33 is represented by the arrow 127 in FIG.4.

Skirt 120 is preferably made from a strong resilient plastic, such asPTFE or Delrin, but may be made from any appropriate material includingother plastics, rubber, and metal, such as stainless steel. To simplifythe manufacturing of skirt 120 and to introduce versatility, skirt 120may be made from two pieces, as shown in FIG. 13 including a lower rampring 128 and an upper conical sleeve 130. Ramp ring 128 and conicalsleeve 130 may be formed separately (milled or molded) and thereaftersecured along a mating surface 132 to each other using any appropriatebonding, fastening or welding technique, as understood by those skilledin the art. In one embodiment, the two pieces making up skirt 120 areattached in an easily removable manner so that one of many conicalsleeves 130 having a particular angle A may be fitted with one of manyramp rings 128 having a particular angle B. Although it is preferredthat skirt 120 be provided as a part to be attached to the lower portionof partition 24, it is also contemplated that skirt 120 and partition 24be made integrally as a single piece.

The purpose of skirt 120 is to provide a somewhat horizontal surface(lower surface 126) on which particles 27 may gradually andatraumatically accelerate in the up bed 33. The lower surface 126 isformed between the wall of partition 24 and the angled surface 122, asshown in FIG. 4. Some prior art Wurster machines use a partition thatincludes an outwardly flared lower end. This flared lower end does notdefine or otherwise establish an angled surface 122 (or any horizontalsurface between the down bed and the up bed). The purpose of the priorart flared lower end is to allow particles to be drawn into the up bedwithout “crowding” the nozzle and disrupting the spray pattern. Althoughthe flared lower end of the prior art partition forces particles locatedin the down bed outwardly towards the peripheral wall of the productcontainer, the particles are still traumatically drawn into the up bedbecause there is no angled (or generally horizontal) surface 122, as inthe present invention.

As described above and according to the invention, skirt 120 divertstablets 27 located in the down bed 37 away from lower edge 25 ofpartition 24 and provides an inclined ramp (angled lower surface 126) sothe adjacent tablets 27 are not harshly and traumatically drawn into theup bed 33. Skirt 120 also helps channel the “loading” of down bed onorifice plate 14 outwardly near the periphery of the product containing12. By doing this, a larger transition zone is created. Tablets 27located under skirt 120 are more easily suspended by the air flowingthrough orifice plate 14 from lower plenum 16 because there is less orno downward force exerted on them by tablets 27 located higher in thedown bed 37. The result is that tablets 27 move more easily and lesstraumatically from the down bed 37, through the transition zone, andinto the up bed. To maximize this effect, it is preferred that thedistance between the outermost point of skirt 120 and the wall of theproduct container 12 (represented by arrow C in FIG. 4) be approximatelyequal to the distance between the lowermost point of skirt 120 and theupper surface 26 of orifice plate 14 (represented by arrow D in FIG. 4).

The particular dimensions and shapes of all the above-describedcomponents of the improved machine have a mathematical relationshipwherein the particular parameters of one component are related anddetermined by the particular parameters of another component, and thecharacteristics of the particles being coated, and the desired coatingresults. Finite analysis techniques may be used to establish therelationship between the components.

5. Central Discharge

A rate limiting factor in the productivity of the Wurster process is therelatively narrow coating zone which, in turn, restricts the diameter ofthe partition 24 to about nine inches. To increase the efficiency andthe productivity of a Wurster machine, the size of the product containermay be increased if multiple partitions and nozzles are used. Forinstance, to operate efficiently, a Wurster machine having an eighteeninch diameter product container uses a nine inch diameter partition.However, a thirty two inch Wurster may require three, nine inch diameterpartitions spaced evenly within the product container, and a forty sixinch Wurster may include six or seven, nine inch diameter partitions.

Referring to FIGS. 18, 19, 20 and 21, an improved product dischargeassembly 140 of the present invention is shown, suitable for efficientlyremoving tablets 27 (or particles, powders, granules, pellets, orgrains) in a sealed and controlled manner from a product container 12 ofthe type having multiple partitions 24 and nozzles 18. An exemplarycoating machine 142 having three nozzles 18 and three partitions 24 isshown and described herein to explain the structure and operation ofdischarge assembly 140, according to the invention. The dischargeassembly 140, according to the invention, may be used with anymulti-partition/nozzle Wurster machine or with product containers inconventional fluidized bed drying or spray granulating equipment. Theabove-described features of the invention including the nozzle ramp, thelower skirt assembly, and the air-discharge manifold are not shown inFIGS. 18 and 19 for clarity. Any and all of the features described inthis specification may be used in any combination in a coating or dryingmachine.

Referring to FIG. 18, partition 24 may include an outwardly flared upperend 145, as shown, having a shape which allows particles 27 to quicklyexit up bed 33 (of partition 24) and enter down bed 37 withoutsubstantially impacting the side wall of partition 24. The specificshape of the flare is preferably cusp shaped, but flared upper end 145may alternatively be conical in shape. Flared upper end 145 allowsparticles 27 to disperse from partition 24 without particle attrition orbreakage and encouraging a smooth transition of the particles from theup bed 33 to the down bed 37. Flared upper end 145 may be used in singlepartition machines, described above, or multi-partition machines, (onlyone of the partitions 27 of FIGS. 18 and 19 is shown with a flared upperend 145 to illustrate the flared feature). Also, the upper end ofpartition 27 may include a resilient or impact absorbent material, suchas a rubber or suitable plastic to help minimize particle or tabletattrition. The absorbent material (not shown in the figures) may be inthe form of a coated layer or an attachable sleeve or layer.

For these larger coating machines, once a coating process is completefor a particular batch of tablets 27, the tablets are typically removedby opening a pivotal lower end of the machine and literally dumping thecontents of the product container 12 into an awaiting container (notshown). As described above, this prior art process for removing coatedtablets 27 may easily introduce contamination to both the tablets andthe interior portions of the machine 10, as well as expose workers topotentially hazardous materials.

The improved machine 142, shown in FIGS. 18 and 19 includes a dischargeconduit 144 which extends from an orifice plate 146 to an accessiblelocation remote from the machine 142. Orifice plate 146 is similar tothe above-described orifice plate 14, except that it is designed forthree nozzles 18 positioned 120° apart from each other and thereforeincludes three large openings (not shown). According to this feature ofthe invention and referring to FIGS. 18-21, orifice plate 146 furtherincludes a central discharge opening 150 which is sized and shaped toaccomodate discharge conduit 144, as shown in FIGS. 20 and 21. A conicalconduit cover 152 is movably fitted above orifice plate 146 in alignmentwith central discharge opening 150. The conical cover 152 is orientedwith its apex directed upward and is movable between two positions, asealed position (shown in FIG. 20) and an open position (shown in FIG.21). A linear actuator 154 is connected to conical cover 152 by one ormore armatures 155 (only one armature 155 is shown in the figures) sothat when activated, actuator 154 linearly displaces armature 155 which,in turn, displaces conical cover 152 between the sealed position (FIG.20) and the open position (FIG. 21). Actuator 154 may be any appropriatetype, such as an electromagnetic actuator (e.g., a solenoid), apneumatically driven cylinder, an hydraulically driven ram device, or amechanically operated device such as a system of cables and/or levers(not shown). The purpose of actuator 154 is to open or close conduitcover 152, as desired, and as further discussed below.

When conduit cover 152 is closed, as shown in FIG. 20, tablets 27 remainsealed within product container 12 and, if coating machine 142 isoperating, tablets 27 will circulate in a manner similar to the earlierdescribed single partition coating machine 10 (represented by arrows 153of FIG. 20), without being obstructed or otherwise affected by conduitcover 152. Conical cover 152 will function as a divider, evenlydirecting the tablets 27 of a central common down bed to each of thethree nozzles 18.

When the coating process is complete (or it is otherwise desired toremove tablets 27 from product container 12), actuator 154 is activatedcausing conduit cover 152 to be vertically displaced above orifice plate146, as shown in FIG. 21, forming a gap 156 between the perimeter ofdischarge opening 150 and the perimeter of conduit cover 152. Theopening of conical cover 152 exposes an open end of discharge conduit144 which causes tablets 27 to be drawn into discharge conduit 144, asrepresented by arrows 158 of FIG. 21, as they continue to circulatewithin the coating machine 142 between the down bed and the up bed, asdescribed above in connection with earlier features of the invention.Tablets 27 are preferably drawn into discharge conduit 144 using apressure differential between the product container 12 and the dischargeconduit (i.e., by creating a vacuum within discharge conduit 144). Thetablets 27 drawn into discharge conduit 144 (represented by arrow 160 ofFIG. 21) exit product container 12 and may be collected in an awaitingcontainer (not shown), while remaining in a sealed and controllableenvironment.

The central discharge assembly described above and shown in FIGS. 18-21,is intended to be used only for larger coating machines which requirethree or more nozzles 18 and partitions 24. The central dischargeassembly is preferably located in the center of the product container12, but could be located elsewhere along orifice plate 14. The centraldischarge feature of the invention may be used alone or in combinationwith any of the other features of this invention and further with anyconventional coating machine, or other type oftablet-processing/handling machine including fluidized bed granulatingand/or drying machines. The central discharge assembly is preferablyused in combination with the above-described compressed air manifoldwhich would assist in forcing product located along the periphery of theproduct container 12 inwardly towards the central discharge assembly.This assistance to the tablets is particularly useful near the end ofthe discharge process when few tablets remain in the product containerand the fluidization air has been reduced to a minimum or stoppedcompletely. The central discharge system is not limited to use withtablets, but is equally effective with smaller substrates such aspellets, granules, crystals or powders.

6. Cleaning

After a predetermined period of cycle time, the interior surfaces of themulti-partition coating machine 142 described above and shown in FIGS.18-21 must be cleaned using a cleaning fluid. As described above, it isknown to position spray nozzles within the expansion chamber and productcontainer and apply a cleaning fluid along most of the interior wallsurfaces, rinsing drug residue (or other materials) and othercontaminants down towards the orifice plate 14. However, much of thedrug residue includes relatively large particles which are too large topass through the openings 30 of orifice plate 14 or through a finescreen (not shown), if one is used in combination with the orifice plate30. These large particles wash down the wall surfaces of the productcontainer and become deposited onto upper surface 26 of the orificeplate 14 or fine screen, typically in the dead zone 40 (see FIG. 2 PRIORART), while the waste cleaning fluid passes through the fine screen andthe lower plenum 16 as it drains.

In accordance with another feature of the present invention, referringto FIGS. 3, 7 and 18-21, during the cleaning process, cleaning fluid maybe injected under high pressure through inlet conduit 64 and innerconduit 62 so that the fluid discharges through openings 60 and acrossorifice plate 14 (or across a fine screen, not shown, which may beplaced on top of orifice plate 14). The radially discharged cleaningfluid will force any of the large particles deposited on orifice plate14 to also move radially inwardly towards the central dischargeassembly. According to the invention, conical cover 152 is moved to itsopen position (as shown in FIG. 21) during the cleaning process so thatthe now exposed central discharge conduit 144 may serve as a centraldrain for any of the larger particles unable to pass through theopenings 30 of the orifice plate.

While the use of the compressed air manifold in combination with thecentral discharge is described for use with a Wurster tablet coatingmachine, these components may also be installed and used in the samemanner for removing powders, granules, and/or coated particles fromconventional fluidized bed drying and/or spraying granulating equipment.Also, each of the above-described improvements of this invention may beused alone or in any combination with each other in any type ofvertical-spray fluidized bed granulating machines or drying granulatingmachines, if appropriate.

7. Split Plenum Arrangement

According to another embodiment of the invention, referring to FIG. 22,a Wuster machine 10 is shown similar to the one shown in FIG. 3 anddescribed above, however the lower plenum 16 now includes a slit-plenumarrangement. The split plenum arrangement includes a central conduit 170and a peripheral conduit 172. The central conduit 170 extends from thelower surface of orifice plate 14 and is sized and shaped to generallydirect air or gas upwardly through nozzle sleeve 90, nozzle ramp 102(through passages 116) to define the up-bed flow of air in the productcontainer 12. The central conduit 170 is connected to a source of airflow (pressurized gas or appropriate fan) not shown, and furtherincludes a metering system 174 for measuring the speed, pressure,volume, humidity, and/or temperature of the passing central up-bed airflow. The metering system 174 is connected to the source of air flow(not shown) so that an air flow having desired flow characteristics canbe achieved and maintained using conventional feedback controllingsubsystems, for example.

The peripheral conduit 172 is sized and shaped to supply air flowthrough the remaining exposed portion (everything around the centralconduit) of the orifice plate 14, thereby controlling the fluidized bedcharacteristics of the down-bed and the transition bed of the productcontainer. Similar to the central conduit 170, the peripheral conduit172 is connected to a dedicated source of air flow (not shown, andsimilarly includes a metering system 176 for measuring the speed,pressure, volume, humidity, and/or temperature of the passing peripheralair flow. The metering system 176 may be similar to the central air-flowmetering system 174 and may similarly be used as a feedback controllingsystem to maintain air-flow having desired preset characteristics.

By separating the flow through the lower plenum 16 into central andperipheral regions, up-bed, down-bed, and transitional-bed flow andfluidization characteristics may be more accurately and independentlycontrolled.

What is claimed is:
 1. An improved Wurster-type fluidized bed apparatusfor applying a coating liquid onto the surface of particles, saidapparatus being of the type including a vertically disposed cylindricalproduct container having a peripheral wall, at least one cylindricalpartition defining a centrally located up bed region and a peripherallylocated down bed region), said product container further including anupper end connected to an expansion chamber, a lower end including anorifice plate having a plurality of openings for passage of fluidizedair, a nozzle centrally located through said orifice plate and beingadapted to generate a spray of said coating liquid upwardly into said upbed, wherein said particles located within said product containercirculate upwardly through said partition and said spray of coatingliquid, between said up bed and said down bed, said improvementcomprising: a generally cusp shaped ramp surface centrally positionedaround said nozzle and directed upwardly towards said partition, saidcusp shaped ramp surface being shaped to direct particles movinggenerally horizontally across said orifice plate from said down-bedupwardly into said partition and said up-bed.
 2. The improved Wurstertype particle coating apparatus according to claim 1, further comprisinga plurality of vertically directed air passages which align with saidopenings of said orifice plate so that a portion of said fluidized flowof air passes through said air passages and creates a cushion of airalong said cusp shaped ramp surface, said cushion of air directingparticles upwardly from said down bed and preventing impact with saidnozzle.
 3. The improved Wurster type particle coating apparatusaccording to claim 1, further comprising a nozzle sleeve centrallypositioned around said nozzle, the nozzle sleeve comprising a generallycylindrical base portion having a circular bottom edge, an upper portionhaving a circular upper edge defining an upper opening, and a centralhollow passage. 4.The improved Wurster type particle coating apparatusaccording to claim 3, wherein said upper portion is a truncated conicalupper portion.
 5. The improved Wurster type particle coating apparatusaccording to claim 4, wherein the surfaces of said nozzle ramp andtruncated conical upper portion of said nozzle sleeve are generallyaligned to reduce the arc of the path traveled by tablets traversing theramp surface.
 6. The improved Wurster type particle coating apparatusaccording to claim 3, wherein said nozzle ramp and said nozzle sleeveare integrally formed.